A major cause of vibration in a single cylinder engine is piston reciprocation. The piston is started and stopped twice during each rotation of the crankshaft, and reactions to the forces that accelerate and decelerate the piston are imposed upon the engine body as vibration in directions generally parallel to the piston axis. When operated in a device such as a lawn mower, the engine produces vibrations that can be transmitted through the device to the operator. This vibration is uncomfortable and could produce operator fatigue. Even when operated in a device in which there is no issue of operator fatigue (e.g., sump pumps or portable generators), engine vibration is undesirable because it causes maintenance problems and tends to reduce the useful life of the machine.
To some extent such vibrations can be decreased by providing the engine with a counterweight fixed on its crankshaft, and located at the side of the crankshaft axis directly opposite the crankpin by which the piston (through the connecting rod) is connected to the crankshaft. Such a crankshaft counterweight arrangement, in which the counterweights rotate with the crankshaft, can be designed to cancel some or even all of the primary acceleration and deceleration forces on the piston assembly along the piston axis. However, the centrifugal force of such rotating crankshaft counterweights also has a component transverse to the piston axis that produces lateral vibration, the amount of which increases in direction proportion to the degree to which the crankshaft counterweights successfully cancel out the acceleration and deceleration forces on the piston assembly. For this reason, many single cylinder engines incorporate crankshaft counterweights having a mass that provides a condition of about “50% overbalance”, such that the centrifugal force due to the counterweights has a component along the piston axis that is equal to about 50% of the acceleration and deceleration forces on the piston assembly. Yet the use of such counterweights having a 50% overbalance condition does not fully eliminate the undesirable vibration occurring in single cylinder engines.
For this reason, additional techniques have been employed to further reduce such vibration. A number of these techniques employ one or more additional counterweights that, in contrast to the crankshaft counterweights discussed above, do not rotate with the crankshaft but instead either reciprocate (that is, move back and forth in a strictly linear manner) with respect to the crankshaft or alternatively move back and forth relative to the crankshaft while also rotating somewhat (in a “wobbling” motion). Such counterweights often are coupled to the crankshaft by way of connecting arms, which have near ends coupled to the counterweights and far ends coupled to the crankshaft. In order that these counterweights move back and forth relative to the crankshaft in opposition to the motion of the piston, circular apertures at the far ends of the connecting arms are supported by eccentric journals on the crankshaft. As the crankshaft rotates, the eccentric journals rotate such that the centers of the eccentric journals rotate about the central axis of the crankshaft, and consequently the far ends of the connecting arms also move around the central axis of the crankshaft. This causes the near ends of the connecting arms, and the counterweights attached thereto, to move back and forth relative to the crankshaft.
Certain existing designs employing this second type of counterweight are configured to produce counterweight movement that includes both movement of the counterweight back and forth relative to the crankshaft as well as additional rotational movement relative to the crankshaft. In one conventional design, a pair of counterweights is not only connected to the crankshaft eccentrics by way of a pair of connecting arms, but also is connected the inside surface of one of the walls of the crankcase within which the counterweights are moving by way of a hinged arm that is rotatably coupled to both the wall of the crankcase and to the counterweights. The hinged arm generally serves to guide movement of the counterweights back and forth in relation to the crankshaft. The movement of the counterweights relative to the crankshaft is not strictly linear, however, since the point at which the hinged arm is attached to the counterweights must follow a nonlinear path determined by the radius formed by the hinged arm relative to the point on the crankcase about which the hinged arm rotates.
While the above-described conventional design for a balance system successfully balances to a high degree the forces associated with piston reciprocation (and thus diminishes undesirable vibrations associated therewith), this conventional design has certain disadvantages. In particular, in addition to experiencing motion back and forth relative to the crankshaft, as well as rotational motion about an axis parallel to the crankshaft axis, the counterweights coupled to the crankshaft by way of the connecting arms may also have a tendency during engine operation to experience torquing and even small amounts of rotation about axes perpendicular to the crankshaft axis, something which can result in further undesirable weight shifting and consequent vibrations in the engine. Such motion can result in relative leading/lagging in terms of the relative positions of the connecting arms with respect to the respective eccentric journals on which they ride, something which is exacerbated by the fact that different counterweights are coupled to the different eccentrics. Although the hinged arm coupling the counterweights to the crankcase does to some extent tend to counteract gross movements by the counterweights in these directions, the hinged arm is less able to prevent small movements in these directions, and thus is incapable of preventing some of the undesirable vibrations associated with such movements.
In view of the above discussion, therefore, it would be advantageous if an improved balancing system capable of being used in a single-cylinder internal combustion engine could be developed. In at least some such embodiments, it would be advantageous if the improved balancing system was designed so that any relative leading or lagging experienced in terms of the positioning of different connecting arms on their respective eccentrics was minimized, and so that any movements of the counterweights about axes other than those parallel to the crankshaft were reduced, such that associated weight shifting, vibrations, and other undesirable effects were less likely to occur than in conventional embodiments of balancing systems. It would further be advantageous if at least some embodiments of the improved balancing system were more easily manufactured and/or assembled than many conventional balancing systems.